(1) Field of the Invention
The present invention relates to a video signal converting device, and more particularly to a video signal converting device for converting an analog composite signal into a digital signal.
(2) Description of the Related Art
As efforts have been made in recent years to realize digital TV broadcasts, video signals are undergoing a transition from analog signals to digital signals. Therefore, a growing number of video devices have an interface compatible with digital signals.
Until all video devices are shifted to digital video devices, however, presently prevalent analog NTSC (National Television System Committee) signals still remain in use, and digital video devices need to have a device for converting an NTSC signal into a digital composite video signal (D2 signal).
For converting an NTSC signal into a D2 signal, a color burst signal (fsc=3.579545 MHz) is extracted from the NTSC signal, and the frequency of the color burst signal is quadruplicated (4 fsc=14.31818 MHz). Then, the NTSC signal is sampled at the quadruplicated frequency of the color burst signal, producing a D2 signal.
There has been proposed a sampling clock generator for multiplying the frequency of a clock signal that is in synchronism with a synchronizing signal such as a horizontal synchronizing signal that is applied from an external source, and thereafter frequency-dividing the resultant signal into a frequency-divided clock signal whose reference phase depends on a positive-going edge or a negative-going edge of the synchronizing signal. The proposed sample clock generator has a delay circuit which is not required to be adjusted, and hence can be mass-produced. For details, reference should be made to Japanese laid-open patent publication No. 63-81517, for example.
Another sampling clock generator for automatically making adjustments to bring a reference clock signal into phase with a video signal has also been proposed. For details, reference should be made to Japanese Unexamined patent publication No. 6-282349, for example.
However, if the analog composite signal to be converted contains much jitter, then since the cycle time of a one-line period of the analog composite signal varies, the sampling phase of a digital signal in the one-line period also varies. Therefore, the analog composite signal cannot be converted into a proper digital signal.